1. Field of Invention
This invention relates in one aspect to improvements in electrical power supplies and power supply techniques, and more particularly, to improvements in electrical power supplies and techniques to supply differential positive voltages to loads with reduced common mode voltages. This invention relates in another aspect to improvements in mass data storage devices and methods for operating same using the improved positive differential voltage power supply.
2. Relevant Background
Although this invention is described in the environment of mass data storage devices, it should be noted that the application of the invention has broader applications. In particular, the invention may be used in any application in which a positive only supply voltage is required. Still more particularly, in many differential amplifier or differential architecture applications, usually both positive and negative supply voltages are required to properly bias the amplifier. If positive only voltages are supplied, in the past, the common mode voltage (the average of the differential input voltages) was unacceptably high. If the common mode voltage is too high, in the mass data storage device environment, spikes and other undesirable signal aberrations may occur.
Mass data storage devices include tape drives, as well as hard disk drives that have one or more spinning magnetic disks or platters onto which data is recorded for storage and subsequent retrieval. Hard disk drives may be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Applications for hard disk drives still being developed, and are expected to further increase in the future.
Mass data storage devices may also include optical disks in which the optical properties of a spinning disk are locally varied to provide a reflectivity gradient that can be detected by a laser transducer head, or the like. Optical disks may be used, for example, to contain data, music, or other information.
In some mass data storage devices magneto-resistive heads are employed. A magneto-resistive head is a data transducer that changes resistance when it is exposed to changes in magnetic fields in proximity to the head. A bias voltage is applied to the head, usually by a differential amplifier, and, ideally, the common mode voltage is set at or near ground, or zero volts. However, in the past, this has not been practically achievable.
What is needed, therefore, is a circuit and technique for providing a differential, positive only power supply that can be used, for example, to bias an MR head of a mass data storage device, with a common mode voltage is set at or near ground, or zero volts.
In light of the above, it is, therefore, an object of the invention to provide a circuit and method for providing a positive voltage supply to a preamplifier.
It is an advantage of the invention that the circuit and method presented eliminate the need for an expensive negative voltage regulator.
It is another advantage of the invention that common mode head bias in mass data storage device implementations can be kept at relatively low levels.
It is another advantage of the invention that a positive only power supply that can be provided, for example, to bias an MR head of a mass data storage device, with a common mode voltage is set at or near ground, or zero volts.
According to a broad aspect of the invention, a circuit is presented to provide positive biasing voltages to a biased element, such as an MR head in a mass data storage device, or the like. The circuit includes upper and lower driver transistors to respectively bias respective opposite ends of the biased element with positive voltages. A feedback circuit controls a lower voltage of the positive voltages to be a value as close as possible to a saturation voltage of the lower driver transistor, without causing the lower transistor to saturate.
According to another broad aspect of the invention, circuit is presented to provide positive biasing voltages to a biased element. The circuit includes a differential amplifier having first and second driver transistors connected in series on respective opposite sides of the biased element between a positive voltage supply and a ground potential. An upper current mirror circuit is connected such that the first driver transistor mirrors a fixed drive current through the biased element. A lower mirror circuit is connected such that the second driver transistor mirrors a variable reference current therethrough. A feedback circuit is connected to control the variable reference current to maintain voltages across the second transistor that are just above a saturation voltage thereof.
According to yet another broad aspect of the invention, a circuit is presented to provide positive biasing voltages to a biased element. The circuit includes means for biasing the biased element. The means for biasing the biased element includes first and second switching means connected in series on respective opposite sides of the biased element between a positive voltage supply and a ground potential and means for mirroring a fixed drive current through the biased element. Means are provided for mirroring a variable reference current in a second switching means. Feedback means are connected to control the variable reference current to maintain voltages across the second switching means that are just above a saturation voltage of the second switching means.
According to still another broad aspect of the invention, a mass data storage device is provided. The mass data storage device includes an MR head and a differential amplifier having first and second driver transistors connected in series on respective opposite sides of the MR head between a positive voltage supply and a ground potential. An upper current mirror circuit is connected such that the first driver transistor mirrors a fixed drive current through the biased element. A lower mirror circuit is connected such that the second driver transistor mirrors a variable reference current therethrough. A feedback circuit is connected to control the variable reference current to maintain voltages across the second transistor that are just above a saturation voltage thereof.
According to yet another broad aspect of the invention, a method is presented for providing positive biasing voltages to a biased element. The method includes biasing respective opposite ends of the biased element with positive voltages, and feeding back a lower voltage of the positive voltages maintain a the lower voltage at a value as close as possible to a saturation voltage of a transistor for biasing the biased element, without causing the transistor to saturate.